1. Field of the Invention
The present invention relates to an optical disc device. More particularly, it relates to an optical disc device having the function of position control of a carriage and a lens actuator.
2. Description of Related Art
Optical disc devices are constructed to emit laser light from a semiconductor laser, to focus it through an object lens into a minute spotlight (hereafter, referred to as a light spot) and to apply the light spot onto a disc surface so that information can be written and read on the disc surface. For example, in the case of a magneto-optical disc, a spiral guide groove for guiding the light spot is formed in the disc surface so that information can be recorded on a convex portion (land) formed between the adjacent guides. The land corresponds to a track on which information is recorded. One round of the track is divided into a plurality of sectors (units for information recording).
In controlling an access of the light spot in recording and reproducing information to a track formed on the disc, first the light spot is moved to a designated track, then address information is read for each of the sectors of the designated track so as to designate a location at which the information is to be recorded or reproduced (sector location), and then the light spot is positioned at the designated location.
An operation that moves the light spot to a target track formed on a disc as described above is called a seek operation.
There have been proposed optical disc devices in which a constitutive block for generating the light spot is mounted in a carriage in order to reduce the size and profile of the optical disc devices. For example, optical elements such as a laser diode for emitting light beams, an object lens for focusing the light beams on the optical disc, a prism and the like; focus-error-signal detecting members for detecting a focus error signal (FES); tracking-error-signal detecting members for detecting a tracking error signal (TES); a lens actuator for slightly moving the object lens; and the like member are mounted in a carriage movable in a radial direction of the disc.
In such optical disc devices, seek control is performed in two stages by a rough seek operation and a precision seek operation. The rough seek operation is a seek operation that moves the entire carriage by a large distance in the radial direction of the disc to bring a light spot near to a target track. The precision seek operation is a seek operation that moves the object lens slightly in the radial direction of the disc by the lens actuator to set the light spot precisely to the target track.
The rough seek operation employs a stepping motor or a DC motor to move the carriage linearly via a lead screw or a gear. In many cases, in order to reduce the size and profile of optical disc devices, the rough seek operation employs a combination of a stepping motor and a gear.
By mounting the carriage over a linear screw-threaded transmission member (gear) and transmitting a rotational force of the stepping motor to the transmission member so as to rotate the transmission member, the carriage is moved linearly in the radial direction of the disc.
In general, upon receipt of a single pulse signal (drive pulse), a stepping-motor control section causes the stepping motor to be driven according to a predetermined pattern of excitation and thereby to be rotated by a given angle θ0, with the result that the carriage is moved linearly by a given distance L0 responsive to the angle θ0. That is, the carriage is moved with the distance L0 as a unit distance, and cannot be moved by a distance smaller than the distance L0. Thus, the lens actuator is used for slightly moving the object lens in order to achieve accurate tracking.
The lens actuator includes a pair of electromagnets disposed in the vicinity of the object lens for controlling the position of the object lens by attraction forces of the electromagnets. The object lens is located at a “neutral position” serving as a so-called reference if current is not applied to the electromagnets, whereas it is displaced by the attraction forces of the electromagnets to a position slightly off the neutral position if current is applied. The object lens, which is urged by a spring, returns to the neutral position by a restoration force of the spring if the application of current is stopped.
In the case where eccentricity is absent in the optical disc, the light spot can be set to the target track with a high accuracy owing to the aforementioned two-stage seek control. Once being set to the target track, the light spot does not deviate from the target track.
However, in general, in the case where the eccentricity is present in the optical disc, the light spot deviates from the target track to the right or left during one turn of the optical disc, regardless of whether the optical disc has a spiral guide groove or coaxial tracks.
Against this, there is provided a circuit that, using light reflected from the optical disc, detects a signal responsive to an amount of deviation of the light spot from the target track (a tracking error signal (TES signal)). The circuit is called a tracking-error-signal detecting circuit. By displacing the position of the object lens in the carriage by the lens actuator based on the TES signal, the light spot is automatically adjusted to the target track on the optical disc so as to follow the eccentricity.
Many techniques have been proposed in which the eccentricity of the optical disc is detected and then a correction is made to compensate for the eccentricity for achieving an accurate seek control (for example, in Japanese Unexamined Patent Publication Nos. HEI 5(1993)-109101 and HEI 5(1993)-144051).
Also, optical disc devices have been proposed in which the eccentricity of the optical disc is learned in advance utilizing the tracking error signal (TES) and then, at the time of recording and reproduction on the optical discs, tracking control and the like are made using the eccentricity information obtained through the learning (for example, in Japanese Unexamined Patent Publication Nos. 2000-339729 and HEI 11(1999)-86309).
Meanwhile, in the case where the eccentricity is present in the optical disc, as described above, the position of the object lens is displaced slightly by the lens actuator. However, there is a limitation on the amount of displacement of the object lens. If the eccentricity exceeds a given maximum amount of displacement (hereafter, a slice), the object lens is displaced, and the carriage is also moved in a direction of displacement of the object lens by driving the stepping motor, since it is not preferable to make the light spot follow the eccentricity only by displacing the object lens by the actuator.
In the case where the eccentricity is present in the optical disc, the optical spot, if it once deviates off the track to the left, possibly deviates in the reverse direction to the right during one turn of the disc, making it necessary to move (return) the carriage in the reverse direction. Thus, there may be a return deviation opposite in direction to the initial deviation depending on the amount of the eccentricity, possibly making it necessary to drive the stepping motor not only once but twice during one turn of the disc.
While such driving of the stepping motor to reciprocate the carriage is an operation needed for accurate tracking, it increases power consumption since it is made inevitably every turn of the disc.
Further, since such driving of the stepping motor is made for a short period of several microseconds, it provides vibrations to the lens actuator, and affects tracking performance.
In the case where the object lens is urged by the spring, during driving of the lens actuator, the returning reaction of the spring generates disturbance in tracking, and possibly decreases the stability in tracking servo.